1. Field of the Invention
This invention relates to a protective relaying system, and more particularly to a protective relaying system which protects a power system on the basis of terminal information quantities received at respective electric stations and reciprocally communicated by means of optical transmission.
2. Description of the Prior Art
ln recent years, the technology of utilizing optical communication lines supported by optical fibers as information transmission routes has pervaded even the field of protective control engineering. Signal transmission by means of optical transmission has significant advantages such as freedom from adverse environmental influences, particularly from electromagnetic disturbances. Thus, applications to protective relaying systems have been projected in which highly reliable information transmission is indispensable. This projected system utilizing optical transmission will be described with reference to FIG. 1 illustrating the skeleton diagram thereof. Although the diagram is illustrated for an electric station A, a similar structure is also used for an electric station B.
In FIG. 1, characters S.sub.A and S.sub.B represent background power sources of the 1st and 2nd electric stations A and B, respectively. Currents I.sub.A and I.sub.B which flow into the terminals of the respective 1st and 2nd electric stations A and B connected by means of a transmission line 1 are respectively converted by current transformers 2.sub.A and 2.sub.B into signals S.sub.0 and S.sub.0 ' suitable to be processed in the following protective circuits. The output signal S.sub.0 from the current transformer 2.sub.A is fed into a protective relay 3.sub.A. The protective relay 3.sub.A is a failure district detection relay that receives the electrical quantities of both the terminals of the transmission line 1 and operates when a failure occurs on the transmission line 1 to be protected (to be more strict, on a portion of the transmission line inside the installation point of the current transformers 2.sub.A and 2.sub.B). The protective relay 3.sub.A is, for example, a current differential relay or a phase comparison relay. A current differential relay will be described herein. The protective relay 3.sub.A utilizes the signal S.sub.0 for operation judgment and also transmits it through an optical transmission line 4 to the opposite terminal. To perform this optical transmission, the protective relay 3A converts the signal S.sub.0 into a signal S.sub.1 so as to meet the specified input condition of a PCM (pulse code modulation) optical terminal device 5.sub.A, and outputs the signal S.sub.1 to the optical terminal device 5.sub.A. The optical terminal device 5.sub.A mixes the signals S.sub.1 so as to produce a multiplex signal, and, in turn, converts the multiplex signal into an optical signal S.sub.2 for outputting the optical signal S.sub.2 to an optical transmission line 4. Further, the optical terminal device 5.sub.A converts an optical signal S.sub.3 received through the optical transmission line 4 from an electric station B into an electrical signal, and also separates the multiplexed electrical signal so as to produce a signal S.sub.4, then outputs the separated signal S.sub.4 to the protective relay 3A. The signal S.sub.4 is also fed into an error detection circuit 6.sub.A which will later be described in detail. The protective relay 3A converts the received signal S.sub.4 into the specified signal level therein and compares this converted signal with the signal S.sub.0 of its own electric station so as to judge whether to operate with respect to the instantaneous value of the current. When the protective relay 3.sub.A judges whether it is to operate, i.e., if there exists a failure on the transmission line 1, it then produces an output signal S.sub.5 which is a trip-enabling signal.
On the other hand, the error detection circuit 6.sub.A receives the signal S.sub.4 so as to monitor whether abnormalities exist in the signal S.sub.4, that is, in the optical transmission line 4, and when any fault is detected, the circuit 6.sub.A outputs a relay lock signal S.sub.6 that inhibits the relay output signal S.sub.5. An inhibit circuit 7.sub.A receives the relay output signal S.sub.5 and the relay lock signal S.sub.6, and in case the relay output signal S.sub.5 is "1" while the relay lock signal S.sub.6 is "0", i.e., when the circuit 7.sub.A establishes logical output "1", then outputs a trip output signal S.sub.7 so as to trip a power circuit breaker 8.sub.A. Thus, in this system, when the error detection circuit 6.sub.A detects abnormalities in the signal S.sub.4, the circuit 6.sub.A outputs the relay lock signal S.sub.6 as "1" to the inhibit circuit 7.sub.A so as to inhibit the trip of the power circuit breaker 8.sub.A (a so-called trip-lock).
The above-described protective relaying system are provided as in the following prior art.
1. The technology disclosed in the transactions No. 1081 of the lnstitute of Electrical Engineers of Japan in 1981 nationwide convention entitled "FM carrier current differential relay by means of optical transmission".
This discloses the system of a so-called FM carrier current differential relay in which the input and output signals S.sub.1 and S.sub.4 of the protective relay 3 in FIG. 3 are such FM (frequency modulation) signals that the carrier waves thereof are modulated by the system current signals, and a PCM 24 terminal device is utilized as an interface of the optical terminal device 5.sub.A. For the operation judgment in the protective relay 3.sub.A, analog signals are used, and the error detection circuit 6.sub.A monitors abnormalities of the FM signals on the basis of either detection of out-range frequencies or comparison of periods of FM signals adjacent with one another, so as to detect abnormalities in the signal transmission system.
2. The technology disclosed in Japanese Patent Publication No. 57-9292.
This discloses the system in which the input and output signals S.sub.1 and S.sub.4 of the protective relay 3.sub.A are of signals digitally coded, and sampling of these signals is performed in synchronism with each other at the respective electric stations, and then the digitally coded current information are reciprocally transmitted. As for the error detection method in the error detection circuit 6.sub.A, there are utilized well-known methods such as level-lowering detection, parity check, cyclic code detection, and dual-simultaneous reference.
3. The technology disclosed in the transactions No. 1083 of the Institute of Electrical Engineers of Japan in 1981 nationwide convention entitled "Current differential protective relaying system utilizing optical PCM transmission".
This discloses the system in which the current information of a sub-electric station is digitized and optically transmitted to the main electric station. On the other hand, in the main electric station, the digital signals are converted into analog signals, and by utilizing the analog signals, the operation judgment based on the current differential protection method is performed. The result of this judgment is transmitted to the subelectric station by means of optical transmission. In the error detection circuit 6, failures in optical transmission are detected on the basis of the signal pattern tests and the detection of a signal lose.
In either system of the above-described conventional protective systems, when a failure of signal transmission system occurs in the transmission process, the output of trip circuit is inhibited, and this prevents mistrip of the interrupter even when the protective relay is erroneously operated by the effect of failures in the relay input information. This preventive procedure solves the problem as to a temporary failure of signal transmission system, however, there still exist problems with continuous failures in the information transmission routes such as damage or burn-out of optical cables.
Such problems include, as described with reference to FIG. 1, that when a failure continues, then the trip-lock also continues. Thus, should a system failure occur during this trip-lock, the elimination of such system failure inevitably depends upon the back-up protection. However, the failure elimination by virtue of power circuit breaker-trip which depends upon the back-up protection unintentionally produces a time-delay compared to that performed by a power circuit breaker-trip based on the main protection because the trip is made in a time-limit trip fashion. This time delay in elimination of failure could be hazardous, depending upon the degree of the failures. A distance relay, which is set in a first zone and which is one of the back-up protection relays, sometimes trips instantaneously. However, this distance relay covers approximately 80% of the entire length of the transmission line up to the opposite side bus of a district to be protected. Thus, the system failure in the remaining district of 20%, that is, in the vicinity of the opposite side bus, does not cause the first stage distance relay to be operated.
Furthermore, there are also other problems as follows. At present, such a method has been put into practical use both domestic and abroad, whereby optical fibers for use in optical transmission are incorporated within the overhead ground wires of the transmission lines. The use of optical fiber-compound overhead ground wires need not prepare exclusive routes for optical fibers, so that it is extremely economical. However, in such system structure, in the case of accidents such as steel-tower destruction, fire or destruction by earthquake and the like, the optical transmission is stopped at substantially the same time to the occurrence of the system failure. Therefore, despite the occurrence of the system failure, the relay is unintentionally locked, so that the elimination of the system failure inevitably depends upon the back-up protection relay. Further, in the case of co-installation of optical fibers within the transmission lines, for example, within underground transmission conduit, there also exist the same problems.
Moreover, in optical transmission, such redundant dual structures have generally been employed that optical fibers and optical terminal devices are provided in dual so as to make changeover upon occurrence of failures in the optical transmission, however, this does not serve as a countermeasure in the case of the aforementioned accidents such as steel-tower destruction, fire, or destruction caused by earthquake.